For example, among a refrigeration cycle apparatus of the related art used for refrigeration or air conditioning, there is a type of apparatus that undergoes an expansion process with a positive displacement fluid machine (expansion mechanism), and uses the expansion power recovered at this time for a compression process performed in the positive displacement fluid machine (compression mechanism). A problem encountered in this refrigeration cycle apparatus of the related art is matching of the volumetric flow rate, a so-called “constraint of constant density ratio”. In other words, since the ratio between a suction volume of the compression mechanism that is driven by the recovered power of the expansion mechanism and a suction volume of the expansion mechanism is fixed, when flow rates of both mechanisms are the same, the ratio of specific volumes of refrigerant at inlets of both mechanisms need to match the ratio of the suction volumes.
In the refrigeration cycle apparatus of the related art as described above, for example, an expander is designed under the condition of matching the ratio of specific volumes of refrigerant (the specific volume of refrigerant at the inlet of the expansion mechanism/specific volume of refrigerant at the inlet of the compression mechanism) with the ratio of suction volume (suction volume of the expansion mechanism/suction volume of the compression mechanism). However, when the refrigeration cycle apparatus is actually operated, a gap occurs between the ratio of specific volumes of refrigerant and the ratio of the suction volumes according to a change in condition of the actual operation. In order to match the gap of the ratio of specific volumes of refrigerant and the ratio of suction volumes from the design points, for example, a refrigeration cycle apparatus has been proposed constituted by “a refrigerant circuit in which a compressor 1 having a motor 11, an outdoor side heat exchanger 3, a expander 6, and an indoor side heat exchanger 8 are connected with pipes. Also, a pre-expansion valve 5 is provided on an inflow side of the expander 6. A bypass circuit which bypasses the pre-expansion valve 5 and the expander 6 is provided in parallel with the pre-expansion valve 5 and the expander 6, and a control valve 7 is provided in the bypass circuit. A drive shaft of the expander 6 and a drive shaft of the compressor 1 are coupled, and the compressor 1 uses power recovered by the expander 6 to drive” (for example, see PTL 1).
The refrigeration cycle apparatus of the related art described above (for example, see PTL 1) causes a predetermined amount of refrigerant to flow in the bypass circuit when (specific volume of refrigerant at the inlet of the expansion mechanism/specific volume of refrigerant at the inlet of the compression mechanism)>(suction volume of the expansion mechanism/suction volume of the compression mechanism). At this time, flow rate of the refrigerant to be circulated through the bypass circuit (opening-degree of the control valve provided in the bypass circuit) is controlled based on the bypass flow ratio that is determined by determining the optimum high pressure that maximizes the C.O.P. Also, when (specific volume of refrigerant at the inlet of the expansion mechanism/specific volume of refrigerant at the inlet of the compression mechanism)<(suction volume of the expansion mechanism/suction volume of the compression mechanism), the pre-expansion valve provided on the suction side of the expansion mechanism reduces the pressure to a predetermined pressure and expands the refrigerant flowing into the expansion mechanism.